Autoclaves are known in which a main medium can be defined as a medium by which the intended main function is achieved, and the control medium is defined as having an auxiliary function by which the main function is obtained. In sterilization by heat and moisture the main medium is steam. In sterilization by gas the main medium can be formalin, and in hot air sterilizers it is hot air. In sterilization by gas, the control medium is a heat conveying medium, for example steam.
As stated above, autoclaves of this type are known, but they have drawbacks and are therefore considered unsatisfactory. For example, when the items in the chamber before sterilization are heated for a period of time towards the sterilization temperature, a pressure increase occurs in the packaging, or a temperature increase is caused in the chamber, which may entail impairment of the contents, or its packaging.
A main object of the present invention is to treat such goods in the autoclave which are subject to sterilization at a temperature that is as high as possible, but yet below a temperature at which the packaging material is damaged. One example of such goods are pharmaceutical products encased in plastic ampoules such as polypropylene, which at a comparatively low temperature the softening compounds in the plastic material starts to diffuse out of the material. Moreover, in the hottest part of a packaging which is not in contact with the liquid in the packaging, the plastic will commence shrinking. Furthermore, the packaging is deformed and cracks may appear therein. This situation cannot be tolerated because of the risk of bacteria growing into the aforesaid cracks.
It is desired to work as near as possible to the temperature tolerance of the plastic without exceeding or reaching this temperature, in order to achieve the required sterilization of the products. The material of the packaging can easily be deformed at a high temperature and this material does not tolerate great differences in pressure between the ambient and the contents. Therefore, the exterior pressure must all the time be balanced against the interior pressure of the package. In this connection, it is known to apply a support pressure, for example by using compressed air, for such balancing purposes.
It has been proposed initially to supply only steam for heating in order to achieve the quickest possible heating of the autoclave chamber. The autoclave has a steam supply conduit with a control valve which is dependent on a thermostat whose sensing body is placed in the supply conduit after the valve. The thermostat is then set at a temperature which is above the intended sterilization temperature in the chamber. The vent conduit has a control valve which is dependent upon a pressostat that reacts according to the pressure in the chamber. The pressostat is set so as to maintain a steam pressure corresponding to the sterilization temperature. Therefore, assuming that the sterilization temperature is 108.degree. C., the steam pressure will be 0.4 at. The supply of steam continues also after the intended pressure has been obtained, but simultaneously steam is permitted to flow out through the vent conduit. At the same time, a thermostat, whose sensing body is located in a package or in a test bottle, controls the temperature in the packages. When the temperature has reached a given value, for instance 70.degree. C., the pressure in the packaging commences to rise above the pressure of the chamber. To prevent this interior pressure from deforming the packaging, a support pressure, for example generated by some gas, can be used in the chamber. The most suitable gas with respect to temperature and pressure would be helium or hydrogen, which easily mix with steam. For practical reasons, however, air is used. If it is desired to increase the pressure in the chamber by means of compressed air it has been found that there will not be a uniform momentous distribution of air in the entire chamber, but a series of pockets formed with steam surrounded by air. Instead of obtaining only a partial pressure, the steam pockets will be compressed to the same pressure as the total pressure. Thus, a pressure increasing to a value corresponding to the total pressure, which is about 0.9 at. is obtained in a gas bubble. The temperature of the steam increases from 108.degree. to 118.degree. C.
In order to solve the aforesaid problem it is hereby proposed to blow the steam out of the chamber, instead of supplying a support pressure. The steam is then replaced by a homogeneous mixture of steam and air, which components are mixed in the supply conduit before being introduced into the chamber. In this way, it is possible to achieve a considerable improvement of the conditions in the chamber. However, experience has shown that also by this procedure too high a temperature may occur in the chamber. This is avoided by the teachings of the present invention by means of a short injection of cooling water and the exchange of the main medium for the control medium.